I. Field of the Invention
The present invention is directed generally to autogenous packaged, chemical or electrochemical analytical cell systems. More particularly, the invention involves a self-activating chemical sensor for sensing the concentration of gaseous oxygen in an atmosphere. The oxygen sensor includes a self-activating, hydrophilic outer layer which uses the atmosphere humidity to provide sufficient moisture to a hydratable electrolyte to activate and permit stable operation of the oxygen sensor without the need for pre-conditioning.
II. Description of the Related Art
The field of diagnostic medicine is fast becoming more sophisticated and complex. Instrumentation of the class includes devices that monitor and make critical determinations pertaining to ongoing life-threatening conditions. Hence, a great need exists for devices that reduce the time required to make many of these determinations in order that proper, timely corrective steps may be taken to improve or stabilize the condition of the patient, for example, during surgery or during the treatment of traumatic injury. In this regard the analysis of gases including the partial pressures of oxygen (PO.sub.2) and carbon dioxide (PCO.sub.2) in the blood (blood gas analysis) are examples of extremely important instantaneous indications of respiratory deficiency, efficiency of inhalation therapy, renal function and other vital bodily processes.
Presently, such measurements are made utilizing stationary electrochemical clinical laboratory instruments that have large reference electrodes together with an array of sensors including pH, CO.sub.2 and O.sub.2 sensors. Periodic recalibration and the running of frequent control samples is required. Calibration and operation measurements using the instrument, to be accurate, are generally restricted to a specific known temperature, e.g., 37.degree. C. The reference and pH electrodes must be calibrated using a liquid media of known composition. The CO.sub.2 and O.sub.2 electrodes can be calibrated using either liquid medium or a calibration gas, but liquids here also have restrictive temperature ranges for use. This occurs because the composition of typical liquid control or calibration fluid systems is such that the equilibrium partial pressures of oxygen and carbon dioxide are temperature dependent and so reference values occur only at a specific temperature. Exposing the liquid-based calibration system at a temperature other than that designed may introduce a decided amount of error into the readings. Use of calibration gas, of course, requires provision of cylinders of compressed gas of known composition.
It is readily apparent that a portable blood-gas analysis system, particularly one providing a self-calibrating system which is small, easily portable and ambient temperature independent would offer a great advantage. This could eliminate the need for running control samples and would allow calibration to be made at the time of testing. Rapid results would be available to the attending physician at patientside during a procedure.
One alternate prior approach to oxygen calibration using oxygen dissolved in a liquid medium involves the use of relatively inert fluids which have the ability to dissolve rather large amounts of oxygen and which are stable with respect to biological media. One class of such materials consists of certain fluorinated organic compounds known as perfluorocarbons. Perfluorocarbon based systems may provide stable concentrations of oxygen in the calibration medium despite changes in temperature of the calibration medium or solvent within a reasonable range of ambient temperatures. One such system is illustrated and described in copending application No. 07/806,495, now U.S. Pat. No. 5,223,433 assigned to the same assignee as the present invention.
Another prior approach is disclosed respecting an oxygen sensor contained in a bank of electrochemical sensors housed in a disposable cartridge for sensing a plurality of components in which the oxygen sensor is activated by a hydrating liquid just before use. That oxygen sensor employs a layered structure in which two layers are disposed over Ag/AgCl and platinum electrodes. The lower layer next to the electrodes is a layer of a dried residue of a hygroscopic material containing hydratable saccharide or polysaccharide material and an amount of electrolyte salt such as KC1 and the upper or outer layer is a water and gas-permeable hydrophobic film formed polymeric layer. The sensor is designed to remain in the dry state until pre-conditioned for first use. Just prior to use, an aqueous liquid calibrating solution is caused to reside above the outer layer and water passes by permeation through the outer layer to hydrate the hygroscopic material and activate the sensor cell. While this system is generally successful, it does have several limiting factors. Those include the pre-conditioning time delay required to allow hydration of the hygroscopic material and activation of the system and the requirement that calibration be made using oxygen dissolved in a liquid medium. Furthermore, the preferred hydrophilic materials are ones tending to swell significantly when hydrated which further tends to slow sensor response.
The present invention, on the other hand, involves a simple, stable and accurate three-layer approach to a sensor for the determination of gaseous or atmospheric oxygen that includes a relatively self-hydrating device. Chemical oxygen detectors or sensors of the type typically used for quantitatively sensing oxygen even in a relatively dry gaseous state require electrolyte medium in conducting state medium connecting the electrodes to activate and operate the sensor. The invention allows the requirement to be met by maintaining sufficient humidity in the atmospheric environment of the oxygen sensor, such that gaseous oxygen sensing is stabilized with respect to the storage, calibration and use of the oxygen sensor without need for pre-conditioning.
Accordingly, it is an object of the present invention to provide an oxygen sensor system which is self-activated from the "dry state" using storage atmospheric humidity which is ready to detect oxygen in a gaseous atmosphere without pre-conditioning.
Another object of the invention is to reduce the response time for an electrochemical oxygen sensor which includes an hydrophilic layer in conjunction with a gas-permeable membrane for gaseous oxygen sensing.
A further object of the invention is to provide an oxygen sensor for a self-contained measurement/calibration cartridge having an autogenous calibration capability for pH, CO.sub.2 and O.sub.2 in which the O.sub.2 is calibrated in the gaseous phase and measured from a blood sample.
A still further object of the invention is the provision of a gaseous oxygen sensing electrode system which will withstand introduction of a liquid blood sample without detrimental effects.